FIG. 4 is a sectional view showing a structure of an electric double layer capacitor representing one example of conventional capacitor of this type. FIG. 5 is a sectional view of an element used for the electric double layer capacitor. In FIGS. 4 and 5, reference mark 11 denotes the element, and reference mark 11a denotes a void space formed in element 11. Element 11 is so constructed that a pair of positive and negative electrodes, each comprising a collector made of a metallic foil such as an aluminum foil having a polarized electrode layer formed thereon except for one side of the edges, are arranged in orientations opposite to each other, and they are rolled up (not shown) with a separator interposed between them in such a configuration that bare portions where the polarized electrode layers are not formed along the side edges of the positive and negative electrodes protrude from the opposite sides with respect to each other. The positive and negative electrodes are thus tapped out individually from the respective sides (at the top and the bottom in FIG. 4) of element 11.
Positive electrode termination 11b and negative electrode termination 11c formed of the bare portions not having the polarized electrode layers at both sides of element 11 have same protruding lengths as indicated by H3.
Reference mark 12 denotes a cylindrically shaped metal casing having a bottom and being made of aluminum containing aforesaid element 11 together with an active electrolyte (not shown). Negative electrode termination 11c of element 11 inserted in metal casing 12 is bonded both mechanically and electrically to an inner bottom surface of metal casing 12 by such means as laser welding.
Reference mark 13 denotes a terminal plate made of aluminum disposed to and seal an opening of metal casing 12 while having its inner surface bonded to positive electrode termination 11b of element 11 by laser welding or the like means. Reference mark 14 denotes a seal rubber of electrical insulation property disposed around a peripheral edge on an upper surface of terminal plate 13, and rubber 14 seals metal casing 12 when the top opening is crimped so as to press rubber 14.
The conventional electric double layer capacitor constructed as above has a structure in which positive electrode termination 11b and negative electrode termination 11c formed on the both sides of element 11 are bonded to terminal plate 13 and the inner bottom surface of metal casing 12 respectively. Accordingly, the capacitance thereof are increased by improving an efficiency of using a capacity available for containing the element while also achieving a low resistance by virtue of the structure.
Patent document 1 is one example of the prior art documents known to be relevant to the invention of this application.
Conventional electric double layer capacitors of the above type have a problem, however, that they are still far from satisfying the accelerating demand of further reduction in size although they have attained a substantial reduction as compared with the ordinary type of capacitors.
One of the reasons is that both positive electrode termination 11b and negative electrode termination 11c are fabricated to have the same protruding lengths of H3. If they are not of the same length, one side of the element having a smaller length of the bare portion not provided with a carbon electrode layer is not likely to become buckled, or deformed due to a larger bearing strength (i.e., rigidity or reactive force) whereas the other side of the element having a larger length of the bare portion becomes buckled or deformed easily when the element is sandwiched from both sides between the inner bottom surface of the metal casing and the bottom face of the terminal plate. When there is a difference in degree of buckling between the two sides of the element, the one side of the bare portion exhibiting a larger degree of buckling tends to exert local stresses on the separator. This causes a reduction in distance between the electrodes at areas where the stresses are exerted, and decreases resistances at these areas due to the short distance, which leads to concentration of electric current around these areas when energized, thereby giving rise to another problem of increasing a possibility of becoming a condition near to short-circuiting. The positive and negative electrode have therefore been fabricated to the same dimensions in the past to avoid the above problem. It is also for the same reason that the buckled lengths have been set equal as shown by F3 in FIG. 4.    Patent Document 1: Japanese Patent Unexamined Publication, No. 2006-173440